The present invention relates to the transmission of telecommunication data using Asynchronous Transfer Mode (ATM) protocol and ATM adaptation layers (AAL). More specifically, the invention relates to the transfer of telecommunication data by multiplexing data from a variety of different AAL protocols over a single communication channel.
ATM is a standard protocol that may be applied for transmitting asynchronous telecommunication data within a telecommunication system that may service one or more applications (i.e., one or more sources of telecommunication data). The ATM protocol is based on the transmission of data in fixed size cells known as ATM cells. The protocol for each ATM cell is the same, wherein, each ATM cell contains a 48 octet payload and a 5 octet header. In general, ATM is well known in the art.
The telecommunication data associated with each application is initially in a data transfer format that is specific to that application. If ATM is to be used for transporting this data, the data transfer format must be adapted so that it is compatible with the ATM protocol. This is accomplished by an ATM adaptation layer (AAL) 101, as illustrated in FIG. 1. The application layer 102 represents telecommunication data arriving from a specific application. The AAL 101 then reformats the data so the data is compatible with the ATM protocol. Specifically, the AAL 101 stores the data in the payload of ATM cells. The ATM layer 103 then transmits the ATM cells containing the reformatted telecommunication data to a receiving station (not shown in FIG. 1). In general, each application requires a different AAL.
Two of the most commonly known AALs are designated AAL1 and AAL5. AAL1 is used for synchronous data transmission and AAL5 is used for packet data transmission. Another well known AAL has been designated AALm. AALm is typically employed to make ATM more efficient when ATM is used to support the transportation of low bit rate application data, such as cellular voice data. AALm operates by storing the low bit rate data in small data packets called microcells (also referred to as minicells). The microcells are then multiplexed into the payload of ATM cells.
When the telecommunication system is servicing more than one application at a time, e.g., voice and video, or the system is servicing one application that produces hybrid data (i.e., produces data exhibiting more than one data transfer format), more than one type of AAL will be required to reformat the data to make it compatible with the ATM protocol. Present systems transmit data from multiple AALs to a receiving station by dedicating separate ATM channels for each type of data. This becomes problematic especially when low bit rate applications are involved because ATM does not efficiently transmit low bit rate data (i.e., large portions of the ATM cells carrying low bit rate data go unused). Therefore, transporting low bit rate data over multiple ATM channels only compounds the inefficiency, and bandwidth tends to be very expensive.